This research proposal involves measurements of absorption and secretion by the gastrointestinal tract in human subjects. We study normal people to elucidate physiology, and patients with intestinal diseases to elucidate pathophysiology. We use intestinal intubation with a triple-lumen, nonabsorbable marker technique to measure absorption in specific segments of the intestine. We use a one-meal balance method to measure absorption of dietary constituents. We are especially interested in chronic diarrheal diseases, malabsorption syndromes, mineral and nutrient absorption and malabsorption, antidiarrheal drugs, and fecal incontinence. Our aim is to expand knowledge of pathophysiology and to discover better diagnostic and therapeutic methods for people who suffer from gastrointestinal diseases.